Generally, a solid state image sensing dement has a construction shown in FIG. 1 wherein photodiodes converting received light signals into electronic signals are arranged in matrix form having certain distances in the vertical and horizontal directions, vertical charge coupled device (VCCD) areas for transmitting charges generated in the photodiodes in the vertical direction are formed between the photodiodes, a horizontal charge coupled device (HCCD) area for transmitting charges transmitted from the VCCD areas in horizontal directions is formed at the output ends of the VCCD areas, and a sensing amplifier (SA) for detecting the amount of charge in the HCCD area is formed at the output end of the HCCD area. The operation of the solid state image sensing element is as follows.
The charge generated in the photodiodes by incident light is transmitted to the VCCD area at the moment when a gate bias (15 V) is loaded. The charge transmitted to the VCCD area is transmitted to the HCCD area by a -9 V to 0 V VCCD clocking signal, and the charge transmitted to the HCCD area is transmitted to the output side by a 0 V to 5 V clocking signal, which charge is converted into a voltage signal at the output side to output through the sensing amplifier.
Shown in FIG. 2 is a section of a conventional solid state image sensing element which is fabricated by forming a metal film 11 for light interception on charge transmission and poly gate areas on a substrate photodiode areas 1 and charge transmission and poly gate areas having been formed thereon, forming a metal film in pad portions, forming a metal film 2 for pad by patterning of the metal film in the pad portions, depositing an insulation film 3 such as a nitride film over the metal film 2, carrying out selective removal of the insulation film 3 of the pad portions, forming a flat layer 4 over the substrate, forming color filter layers 5, 6 and 7 on the flat area 4, forming a top coating layer 8 for flattening, forming microlenses 9 each positioned corresponding to each of the photodiodes below on the top coating layer 8, and opening the pad portions by selective removal of the top coating layer 8 and the flat area 4 of the pad portions.
Light incident through the microlens 9 irradiates the photodiodes through the top coating layer 8, the color filter layers 5, 6 and 7 and the flat layer 4. This light, after having been converted into an image signal in the photodiodes and transmitted through the VCCD areas and the HCCD area, is reproduced into an image signal by the sensing amplifier.
As described above, conventional solid state image sensing elements focus incident light by means of a lens array formed on the color filter layers for improving sensitivity. The lens array is formed by forming lens shapes by carrying out patterning of transparent photoresist having a refractive index of 1.6 and melting it.
In such lens arrays, it is preferable to make the distances between the lenses as close as possible because the light incident between lenses is not focused. These distances, in general, are designed to be the same in vertical and horizontal directions (in general, a dimension within the resolution limit of resist). At this time, when the dimensions of a unit pixel (photodiode) in the vertical directions and horizontal directions are different, an image under the lens projected becomes not a regular square but a rectangle (see FIGS. 3(a) and 3(b)). And because curvatures of the lens in the vertical and horizontal sections are different, the focal distances are also different.
Decreased sensitivity of a solid state image sensing element due to such effect cannot be neglected. Consequently, solid state image sensing elements provided with lenses each having a double layer have been proposed.
Referring to FIGS. 3(a) and 3(b), in case the dimension of a unit pixel, i.e., a photodiode 21, in the vertical direction is greater than the horizontal direction, FIG. 3(a) shows a plan view of a situation wherein a stripe microlens and a mosaic microlens are formed in the horizontal direction of the photodiodes. FIG. 3(b) shows a plan view of a situation wherein a stripe microlens and a mosaic microlens are formed in the vertical direction of the photodiodes. And FIG. 3(c) shows a section along line A-A' of FIG. 3(b), wherein the lens is made in a double layer by forming the stripe microlens 32 in horizontal or vertical directions over the photodiode area 21 and subsequent forming of the mosaic microlenses 33 on the stripe microlens 32. The reference numerals 35 in FIGS. 3(a), 3(b) and 3(c) represent focusing points.
Referring to FIGS. 4(a) to 4(f), a method for fabricating a solid state image sensing element adapting an array of lenses having the double layer will now be explained.
First, as shown in FIG. 4(a), a black and white solid state image sensing element 25 having photodiodes 21 and VCCDs 22 is formed, and a nitride film 26 as a passivation layer is formed thereon. The reference number 23 in FIG. 4(a) represents a pad portion.
Then, as shown in FIG. 4(b), a flat layer 27 is formed on the nitride film 26, a first dyeing layer 28 is formed on the flat layer, the first dyeing layer is cured after having been dyed in a first color, followed by forming of a second dyeing layer 29 and subsequent dyeing in a second color and curing then, after a third dyeing layer 30 is formed. The third layer 30 is dyed in a third color and cured, completing the forming of color filter layers 28, 29 and 30. Then a top coating 31 is formed over the substrate.
Next, as shown in FIG. 4(c), after photoresist for forming a stripe microlens layer is coated on the top coating layer 31, exposure, development and reflow are carried out to form a stripe microlens layer 32. In FIG. 4(c), the stripe microlens layer 32 is shown not in partial section but in perspective view to show the round shape of the lens section X.
Then, as shown in FIG. 4(d), after the photoresist for forming a mosaic microlens layer on the stripe microlens 32 is coated, exposure, development and reflow are carried out to form oval mosaic microlens layers 33, each shaped as shown in FIGS. 3(a) and 3(b) and positioned corresponding to each of the photodiodes.
Next, as shown in FIG. 4(e), after photoresist 34 is coated over the substrate, a photoresist pattern 34 for opening the pad portions by selective exposure and development is formed, and the top coating layer 31 and the flat layer 27 are etched to expose the pad portions 23 using the photoresist pattern 34 as a mask.
Then, the forming of a color image scanning element having double layered microlens layers as shown in FIG. 4(f) is completed by removing the photoresist 34.
However, in fabrication of conventional solid state image sensing elements having double layered microlenses, because the mosaic microlenses 33 are formed directly on top of the convex portion of the stripe microlens 32 having a shape of a convex lens as shown in FIG. 3(c), carrying out the patterning and reflow process on the convex surface is not easy.
Moreover, as the degree of integration increases the focusing point or distance should become shorter, which requires shorter curvature of the microlens. However, because, only with the reflow process of the photoresist for forming microlenses in the conventional art described above, decreases of the height together with decrease of the curvature of the stripe microlens cannot be achieved at the same time, application of this art to fabrication of solid state image sensing elements of next generation is not practicable.